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Dell H200 SAS -Obscenely Slow

March 19th, 2014 2 comments

After installing VMware ESXi 5.5 RAID 1 and even RAID 0 performance was obscenely slow with a range of disks.

Much faffing about later and the tldr is basically the card disables the _ON DISK_ cache.

There’s much confusion online thinking the card has some sort of write cache; it doesn’t (though it does deal with SAS drives without requiring a licence (cough HP)). This is a cheap card, no write cache, no battery, none of that jazz (get a PERC 5e if you want some of that excitement). However it’s quite a capable direct sata/sas controller.

Y U go slo?

As default the card disables the cache that’s built into the hard drive itself (you’ll most likely remember it from such columns as (Buffer) Cache on PcPartPicker. I believe the correct term for this would be “on-disk write-buffer”. When looking at a drive for your desktop computer, anything that makes random writes bearable with spinning rust is a boon. If you have an unexpected power-off then the machine will reboot, do a disk check and you might have lost some of your browsing history, or even the last save of your essay… but probably not. Even if you do you’re not going to care.

When you’ve got a server doing mission critical work, when you tell the machine to commit to disk you expect it’s actually on the disk. Think a decent size database (100’s of gigabytes) with lots of sequential access. If you think stuff is safely in longterm storage and (for example) power off whilst it’s still in the cache, you’re likely to have all sorts of hellish corruption. The on disk buffer can also cause all sorts of interesting problems with RAID itself, especially when the disks start dying. To get over the performance hit expensive RAID cards leave the disk buffer disabled and have their own cache (which is usually in the hundreds of MBs if not gig). Known as a Write Cache these are (essentially) sticks of RAM with a battery attached.The card can promise the machine that the data is stored safely before it even spins the disks back up. RAM to act as fast and large temporary storage and the battery so that if the power suddenly dies, all the data in the RAM is still safe and can be written to the actual disks when power returns.

make it fast!

So by now you probably don’t give a damn about securing data integrity and you just want your datastore to be almost vaguely usable. It’s easy. Ish.

Find a Dell Open Manage Server Administrator liveCD or install it on your OS.  If you’re using ESXI then it’s doable but an insane ball-ache.1

Once booted/installed then go into the storage component, into your array and enable the disk cache. Reboot and find it’s almost usable.

-sorry about the lack of full instructions; I did this back in December so at this point you can either have it from memory or it’d just be a saved title in my drafts.

 


  1. you need to install the OMSA VIB on the ESXI host and then find another box to install just the web component server which you can then hit up with a browser. Maybe. It also seems to fail if the web component is on a VM on the same hardware. I will try and make myself go through it again and write some instructions…